2. Field of the Invention
The present invention relates generally to implants, method, and instrumentation for fusion of the human cervical spine from the anterior aspect, and in particular to plate systems for aligning and maintaining adjacent cervical vertebrae in a selected spatial relationship during spinal fusion of those vertebrae.
3. Description of the Related Art
It is current practice in the art to use cervical plating systems for this purpose. Such systems are composed essentially of plates and screws for aligning and holding vertebrae in a desired position relative to one another. The earliest such devices consisted of stainless steel plates and screws and required that the screws passed entirely through the vertebrae and into the spinal canal in order to engage the strong bone tissue (the posterior cortex) of the vertebral bodies. This required the ability to observe or visualize this area radiographically, which is not always possible, especially in the lower cervical spine where the vertebrae may be hidden radiographically by the shoulders.
In order to form holes in the vertebral bodies for insertion of each screw, a drilling operation was performed, followed by a tapping operation. Each of these operations involved the passage of an instrument entirely through the associated vertebral body and into the spinal column. Thus, these instruments come into close proximity to the spinal cord and the dural sac which are in close proximity to the back surfaces of the vertebral bodies. Any procedure which introduces an object into the spinal canal presents serious risks which are of concern to the surgeon.
The conventional technique of forming a bone screw receiving hole in vertebral bodies by drilling has a number of significant disadvantages. For example, drilling removes bone material, leaving a void and resulting in a loss of bone material. Drilling also causes microfracturing of the bone at the drill bit-bone interface and the resulting fracture lines tend to propagate in directions perpendicular to the wall of the hole. More specifically, the bone material is essentially a type of ceramic which exhibits a brittle pattern of fracture formation and propagation in response to drilling. Furthermore, drilling generates heat which can result in thermal necrosis of the bone material precisely at the interface between the bone and a subsequently installed screw, where necrosis is most harmful. Any bone which does experience necrosis will subsequently be resorbed by the body as part of the bone repair process and this can lead to the loosening of the screw.
Another problem with drilling is that the path of the drill bit is difficult to control and since the drill bit operates by rotation, it can wind up soft tissue about the associated plate. In addition, unless great care is taken, the drill bit may be driven significantly past the posterior cortex and cause irreparable harm within the spinal canal. Finally, a drill bit may bind and fracture within the vertebral body and can then cause serious injury as the still rotating portion of the drill bit passes into the wound, while the portion of the bit which has broken off may either protrude dangerously from the vertebral body or may be broken off flush with the upper surface of the body so as to be irretrievably embedded therein. In any event, the steps that must be taken to retrieve the broken-off portion of a drill bit will inevitably prolong and complicate the surgical procedure.
In known plating systems, there have been problems with loosening and failure of the hardware, breakage of the screws and plates, and backing out of screws into the patient's throat area. These occurrences generally require further surgical procedures to replace the broken parts or the plates and screws entirely, and to repair any damage that may have been caused.
Other problems which have been encountered with known systems result from the failure of the screws to achieve a sufficient purchase in the bone and the stripping of the screws.
Also, the use of the known plating systems may result in a loss of lordosis, which is the normal curve of the cervical spine when viewed from the side.
Known plating systems additionally experience problems in connection with those procedures where bone grafts are placed between vertebral bodies to achieve an interbody fusion which heals by a process called "creeping substitution". In this process, bone at the interface between the graft and a vertebra is removed by a biological process which involves the production of powerful acids and enzymes, as a prelude to invasion of the interface by living tissue and the deposition, or growth, of new bone. While the plates allow for proper alignment of the vertebrae and their rigid fixation, they can therefore, at the same time unfortunately, hold the vertebrae apart while the resorption phase of the creeping substitution process forms gaps in the bone at the fusion site with the result that the desired fusion does not occur. Such failure is known as pseudoarthrosis. When such a failure occurs, the hardware itself will usually break or become loosened from the spine, thus requiring a further surgical procedure to remove the broken components and another surgical procedure to again attempt fusion.
In response to the problems described above, a second generation of plating systems has been developed and/or proposed. These include a system disclosed in U.S. Pat. No. 5,364,399 to Lowery and U.S. Pat. No. 5,423,826 to Morscher, as well as cervical spine locking plating systems offered by SYNTHES Spine, the DANEK ORION plate, the CODMAN SHURTLEFF plate, and the SMITH NEPHEW RICHARDS plate, among others. The systems' forming members of this second generation have a number of common properties. They are all made of either a titanium alloy or pure titanium rather than stainless steel, to minimize adverse tissue reactions and are MRI compatible, which stainless steel is not. The screws and the plates have been given increased thickness in order to achieve increased strength. The screws have larger diameters to improve their purchase without requiring that they engage the posterior cortex of the vertebral bodies. Some mild longitudinal contouring of the plates is employed to allow for some lordosis, and/or limited transverse contouring to better follow the generally curved aspect of the front of the vertebral bodies. Mechanisms are employed for securing the vertebral bone screws to their associated plates in a manner to prevent the screws from backing out. While this second generation of plating systems represents a significant improvement over earlier systems, certain existing problems persist, while new problems have been created.
For example, since the screws no longer extend into the posterior cortex, it is common for the threads in the tapped screw hole to become stripped and for the screws to fail to gain a suitable purchase. In addition, screw breakage continues to be experienced and occurs most commonly at the junction of the screw to the posterior aspect of the plate. The screws employed in both the SYNTHES system and the SMITH NEPHEW RICHARDS system are particularly vulnerable to this problem because those screws are hollow at the level where they attach to the plate to permit the internal reception of locking screws.
In an attempt to prevent screw to plate junction breakage of the screw, more recent designs of screws have an increasing root diameter from tip to head, which thus far has resulted in a near useless stubby and blunt thread near the screw head with little holding power and little tactile feedback to the surgeon to signal the completion of tightening prior to stripping of the screw within the bone. Based on empiric studies testing these prior art screws, the use of a pretapped hole, rather than a self-tapping screw, was found to be preferred for pullout strength and thus these screws have not been self-tapping and thus the screw holes must be pre-tapped. Since the thread cutting portion of a tap is necessarily sharp and rotated to work, there is a serious risk of damage to the surrounding soft tissues when it is used. This is compounded by the fact that the plates employed in these systems do not provide sufficient long axis contouring to make full allowance for lordosis and do not have sufficient transverse contouring to prevent rocking of the plate about its longitudinal axis and to conform to the anterior shape of the vertebral bodies, so that these plates do not prevent soft tissue from creeping in from the sides and beneath the screw holes thus exposing these tissues to damage by the drill and the tap. While it is possible, at the time of surgery, to make some change in the contouring of these plates, this is generally limited to contouring of the longitudinal axis and quite often causes distortion of the plate's bone screw holes and screw hole to plate junctions in a manner which has an adverse effect on the screw-plate interlock. Lack of proper contouring prevents these plates from having an optimally low profile relative to the spine.
In some of the second generation cervical plating systems, screw backout continues to occur, because these plates could not be designed to allow for the locking of all of the screws. Specifically, while the designers of these plates recognized the importance of securing the bone screws to the plates, they were unable to lock all of the screws and had to settle for leaving some of the screws unlocked.
Furthermore, several of these second generation systems utilize tiny and delicate "watchmaker" parts to achieve interlocking. These parts are characterized by the need to engage them with particularly delicate small ended screw drivers. These interlocking components are easily rendered ineffective by any effort to alter the contours of a plate during surgery.
Despite the improvement of these second generation plating systems over the first problems, the problems still persist, the most important of which is pseudoarthroses, and particularly "distraction pseudoarthroses". Although these second generation plates have clearly led to an increase in fusion rate, when a failure to produce fusion occurs, it is generally accompanied by bone resorption along a line at the graft-to-vertebra junction, which can be seen on a radiograph.
In the case of the weak first generation plates and screws, the plates might hold the vertebrae apart, preventing fusion, but only until the hardware would break, relieving the distraction, and then allowing the fusion to occur. The second generation systems of plates are too strong to allow this to occur, thus requiring further surgical procedures for the correction of the pseudoarthroses.
Compression plates are well-known and are widely used in orthopedic surgery for the stabilization of tubular bones, and sometimes also flat bones. Such plates may rely on some external compression means or may be self-compressing, relying on the ability of the screw head to slide within a ramped slot such that the tightening of the bone screws through the plate imparts a linear motion perpendicular to the screw axes. U.S. Pat. No. 5,180,381 discloses an attempt to employ such a mechanism in connection with anterior spinal fixation.
However, it has been found that all of the proposed self-compressing plating systems have in common the need for a screw to engage both a proximal and a distal cortex, (bone casing of very dense bone material), so as to anchor the screw tip in a manner to allow the plate to move relative to the screw when tightened rather than allowing the plate to drag the screw off axis. However, as already discussed earlier herein, when a screw is to engage the posterior cortex of the vertebral body, it is necessary for the drill and the tap which form the screw hole, as well as the screw tip itself, to all enter the spinal canal, thereby exposing the spinal cord to damage.
While the system disclosed in U.S. Pat. No. 5,180,381 avoids such danger by engaging the vertebral body end plate instead of the posterior vertebral body cortex, the path of the screw is of necessity quite short, so that there is very little opportunity for the screw threads to achieve additional purchase within the vertebral body. It would therefore appear that to the extent that the device disclosed in U.S. Pat. No. 5,180,380 is able to achieve its stated objectives, it would pull the front of the spine together more than the back and would not appear to compress the back of the vertebral bodies at all, thus producing an undesirable iatrogenic loss of the normal cervical lordosis. Such a situation is disruptive to the normal biomechanics of the cervical spine and potentially quite harmful.
The creation of compression between adjacent vertebrae would offer a number of advantages, including reduced distraction pseudoarthrosis, increased surface area of contact between the graft and vertebrae as slightly incongruent surfaces are forced together, increased osteogenic stimulation, since compressive loads stimulate bone formation, and increased fusion graft and spinal segment stability.
Among the new problems created by these second generation systems is a tendency for the small "watchmaker" parts used to lock the bone screws to the plate to fall off of the driver used for attaching those parts, or out of the associated plates and to become lost in the wound. In addition, these small parts are quite fragile and require specialized additional instruments for their insertion and/or manipulation. Furthermore, incorrect bone screw placement relative to the axis of a plate hole may render the screw locking mechanism unworkable or may cause sharp and jagged shavings of titanium to be formed as a locking screw is driven into contact with an improperly seated bone screw. The means for establishing bone screw to plate hole alignment and preparation are less than reliable. Furthermore, most of these second generation systems lack a reliable and effective means for positioning and holding the plate during attachment.
Specific features of various prior art systems will be summarized below.
The system disclosed in U.S. Pat. Nos. 5,364,399 and 5,423,826, cited earlier herein, includes a thin stainless steel plate which allows for side-by-side or offset bicortical screw placement, the plate having a combination of screw holes and slots.
The "Acromed" system includes a titanium plate and screws which require bicortical screw placement. This system does not include any locking means for the bone screws.
The system disclosed in U.S. Pat. No. 5,180,381 includes an "H" shaped plate having a combination of ramped slots and a hole which requires bicortical screw placement at a 45.degree. angle to the plane of the plate. This patent discloses that this angular positioning is for the purpose of producing compression.
The SYNTHES Morscher plate system employs hollow, slotted screw heads. The screws are placed unicortically so that the heads, when properly aligned, come to rest in the upper portion of the plate holes. The upper portion of each screw is internally threaded to receive a tiny screw which is screwed into the bone screw head in order to increase the interference fit between the bone screw head and the wall of the associated plate hole.
In the system disclosed in U.S. Pat. Nos. 5,364,399 and 5,423,826, use is made of pairs of unicortical bone screws that may be locked in place at both ends of the associated plate by locking screws which have a small diameter shank and a large head. At each end of a plate two bone screws may be locked in place by a single locking screw which is situated between the bone screws. Generally, the plate is provided, between its two ends, with a diagonal slot or slots for receiving one or more additional screws, each additional screw being securable in a bone graft or a respective vertebra which is spanned by the plate. There is no locking screw associated with these intermediate bone screws to lock the bone screws to the plate.
The Codman Shurtleff plating system utilizes the side of a preinstalled rivet having a head rotatable to press against the side of the head of a bone screw so as to secure that one screw to the plate. The plates of this system also are provided with holes for receiving intermediate screws, but these screws are not associated with any locking means.
While the designers of the last-mentioned systems recognized the importance of locking the bone screws in position on their associated plates, they did not provide for any locking of the intermediate bone screws in their associated holes.
In an earlier version of the Codman Shurtleff system, the locking mechanism was a lever pivotable about a shaft passing entirely through the plate and then flared so as to retain the shaft within the plate. The lever was rotated after the bone screw had been inserted to engage the head of the bone screw and thus secure the bone screw to the plate.
Based on a consideration of the features of all of the known cervical plating systems, it appears that there remains a need for an improved system having the following combination of features:
1) The plate should be sufficiently strong to perform its intended function without mechanical failure; PA1 2) The plate should be preformed in three dimensions so as to anatomically conform in both the longitudinal and transverse planes to the anterior cervical spine; PA1 3) The plate should be constructed so that all of the bone screws are generally perpendicular to the plate when viewed from the side, but pairs of screws are highly convergent corresponding to any vertebral level when viewed from the bottom, or on end; PA1 4) Each pair of screws engages in a respective vertebra and the high convergence of screws in a pair allows the length of the screws which engage the bone to be longer and still remain within that vertebra and provide a safer and stronger engagement with the vertebrae; PA1 5) The system should include bone screws which are capable of achieving enhanced purchase within the bone of the vertebral body and without the need to penetrate the posterior vertebral cortex and enter the spinal canal; PA1 6) Use should be made of a screw which is self-tapping, thereby eliminating the need for separate tapping steps; PA1 7) A reliable means should be provided for engaging and manipulating the plate during installation; PA1 8) The plate should be engageable with an instrument means which can reliably produce bone screw holes which are coaxial with the screw holes in the plate; PA1 9) It should be possible to prepare the vertebral bone to receive the bone screws so as to produce a stronger connection and a reduced danger of thread stripping by means of a pilot hole punch creating a pilot hole for the bone screws; PA1 10) Alternatively to the use of a pilot hole punch, a relatively (compared to the overall root diameter of the screw) small diameter drill may be used to create the pilot hole. PA1 11) Means should be provided for locking each and every bone screw in position relative to the plate, and the locking means should be of sufficient size and strength to reliably perform its intended functions; PA1 12) Bone screw locking means should preferably be retainable by the plate prior to bone screw insertion, or should be reliably attachable to a driver to prevent any small parts from becoming loose in the wound; and PA1 13) The system should be capable of effecting compression of the vertebral segments to be fused while maintaining and/or restoring lordosis.